[meteorite-list] Astronomers at Palomar Observatory Discover a 10th Planet Beyond Pluto (Updated)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun Jul 31 22:17:38 2005
Message-ID: <200508010216.j712Gig07100_at_zagami.jpl.nasa.gov>

http://www.gps.caltech.edu/~mbrown/planetlila/index.html

Astronomers at Palomar Observatory Discover a 10th Planet Beyond Pluto

[Images]
Discovery images of the new planet. The three images were taken 1 1/2
hours apart on the night of October 21st, 2003.
The planet can be seen very slowly moving across the sky over the course
of 3 hours.


The planet, with the current temporary name 2003UB313, was discovered in
an ongoing survey at Palomar Observatory's Samuel Oschin
<http://www.astro.caltech.edu/palomarnew/sot.html> telescope by
astronomers Mike Brown (Caltech), Chad Trujillo (Gemini Observatory),
and David Rabinowitz (Yale University). We have proposed a name to the
IAU and will announce it when that name is accepted. For those
speculating that the name proposed is "Lila" based on the web site name
I must warn you that that is really just a sentimental dad's early
morning naming of a web site for his three week old daughter
<http://www.lilahbrown.com> and one should not take it too seriously!

What is it?

This new planet (see "What makes a planet?" below) is the
largest object found in orbit around the sun since the discovery of
Neptune and its moon Triton in 1846. It is larger than Pluto, discovered
in 1930. Like Pluto, the new planet is a member of the Kuiper belt, a
swarm of icy bodies beyond Neptune in orbit around the sun. Until this
discovery Pluto was frequently described as "the largest Kuiper belt
object" in addition to being called a planet. Pluto is now the second
largest Kuiper belt object, while this is the largest currently known.

Where is it?

The new planet is the most distant object ever seen in orbit around the
sun, even more distant than Sedna <../sedna>, the planetoid discovered
almost 2 years ago. It is almost 10 billion miles from the sun and more
than 3 times more distant than the next closest planet, Pluto and takes
more than twice as long to orbit the sun as Pluto.

[Image]
A view of the solar system from the north down. The four circles show
the orbits of Jupiter, Saturn, Uranus, and Neptune. The yellow dot in
the center is the sun.
The earth, if it were shown, would be inside the yellow dot representing
the sun. The orbits of the two outermost planets, along with their
current positions, are also shown.

At the time of the annoucement (late July 2005), the new planet can be
seen high in the morning sky a few hours before the sun comes up in the
constellation Cetus. The planet can be seen using very high-end amateur
equipment, but you need to know where to look. The best way to find
precise coordinates (of this planet, or any other body in the solar
system) is with JPL's horizons <http://ssd.jpl.nasa.gov/cgi-bin/eph>
system. Click on "select target" and then enter "2003 UG313" under
small (!) bodies.

The orbit of the new planet is even more eccentric than that of Pluto.
Pluto moves from 30 to 50 times the sun-earth distance over its 250 year
orbit, while the new planet moves from 38 to 97 times the sun-earth
distance over its 560 year orbit.

How big is it?

Usually when we first discover distant objects in the outer solar system
we don't know for sure how large they are. Why not? Because all we see
is a dot of light, like the top of the page. This dot of light is
sunlight reflected off the surface of the planet (interestingly the
sunlight takes almost a day to get out to the planet, reflect off of it,
and get back to the earth!), but we don't know if the object is bright
because it is large or if it is bright because it is highly reflective
or both. In the case of the new planet, however, we know that even if it
is extremely reflective (like fresh snow, for example) it still cannot
be as bright as it is unless it is bigger than Pluto. Thus while we
don't know for certain the precise size, we know for certain that it is
bigger than Pluto. Here are some interesting guesses to how big it might
be, based on what the surface is like:

Type of surface Amount of light inferred diameter fraction of Pluto's
                     reflected of planet size

nothing known 100% 2210 km 97%
fresh snow on earth 90% 2330 km 102%
average antarctica 80% 2475 km 108%
Pluto-like 60% 2860 km 125%
Charon (Pluto's moon) 38% 3550 km 156%

(we don't think the planet is any bigger than this, based on preliminary
observations from the Spitzer Space Telescope)

Spitzer Space Telescope <www.spitzer.caltech.edu> observations are often
the best way to find the size of objects in the outer solar system. The
Spitzer telescope measures the amount of heat coming from an object. If
we wanted to measure the size of a fire, for example, we could do it by
measuring the total amount of heat coming from the fire. The temperature
of the flames in a match and a bonfire are essentially the same, but a
bonfire emits much more heat because it is much bigger. The same is true
of distant planets. Because we know how far away the planet is we have a
pretty good idea of the surface temperature (a frosty 470 degrees below
zero!), thus when we measure the total heat we can tell how big the
object is. Unfortunately, the new planet is so far away and thus so cold
that even the Spitzer telescope has been unable to detect it (though the
analysis is ongoing; perhaps we will still be able to find the signal).
We know from this, however, that the planet cannot be bigger than about
3550 km. We will refine these numbers as the analysis proceeds.

The next step to try to measure the size will be to observe the planet
with the Hubble Space Telescope <www.stsci.edu> and see if we can do
some very carefull analysis to measure the size in a similar manner as
we did for the planetoid Quaoar. <../quaoar> These observations are
already scheduled and will be taking place shortly.

What is the new planet made out of?

We study the composition of distant objects by looking at sunlight
reflected off of them. The sunlight reflected off the surface of the
earth, for example, shows distinct signatures of the oxygen in earth's
atmosphere, of photosynethic plants, and of abundant water, among other
things. We have been using the Gemini Observatory <www.gemini.edu> on
Mauna Kea, Hawaii to study the light reflected from the surface of the
new planet, and have found that the planet looks remarkably similar to
Pluto. A comparison of the two is shown below, where we show the amount
of sunlight reflected across the near infrared wavelength band. This
type of light, just beyond what is visible to the human eye, is most
sensitive to the types of ices expected on surfaces in the outer solar
system.

[Plot]

The plot above compares the amount of infrared sunlight of different
colors ("wavelength") reflected from the new planet with the amount of
sunlight reflected from Pluto. The dips in the amount of sunlight,
indicated with arrows, are characteristic signatures of a surface are a
characteristic signature of a surface covered with solid frozen methane.
Both Pluto and the new planet show these signatures. At the very low
temperatures of Pluto and the new planet methane, which is in gaseous
form on the earth, is frozen solid. The interior of the planet, like the
interior of Pluto, is likely a mixture of rock and ice.

Pluto and the new planet are not completely identical, however. While
Pluto's surface is moderately red, the new planet appears almost grey.
We are only now begining to try to understand why the colors differ so.

Pluto and the new planet are unlike the other 8 planets. Mercury, Venus,
Earth, and Mars are medium-sized rocky bodies which we call terrestrial
planets. Jupiter, Saturn, Uranus, and Neptune are much larger and called
giant planets. All of these planets are on essentially circular orbits
in a thin disk orbiting around the sun. The two distant planets are a
different type entirely. Both are made of ice and rock, are on very
eccentric orbits, and have their orbits tilted with respect to the rest
of the planets. The new planet is tilted an amazing 45 degrees away from
the rest of the planets!

How was the new planet found?

We have been conducting an ongoing survey of the outer solar system
using the Palomar QUEST camera
<http://hepwww.physics.yale.edu/quest/palomar.html> and the Samuel
Oschin Telescope <http://www.gps.caltech.edu/%7Echad/quaoar/p48.jpg> at
Palomar Observatory <http://www.astro.caltech.edu/palomar/> in Southern
California. This survey has been operating since the fall of 2001, with
the switch to the QUEST camera happening in the summer of 2003. To date
we have found around 80 bright Kuiper belt objects.
To find objects, we take three pictures of a small region of the night
sky over three hours and look for something that moves. The many
billions of stars and galaxies visible in the sky appear stationary,
while satellites, planets, asteroids, and comets appear to move. The
image below shows the three frames taken the night of October 21st, 2003
where we found the new planet. Can you find the moving object?

The area of sky show here is approximately 0.015% of the amount of sky
that we look at every night, but even though we survey vast regions of
the sky per night, it is still going to take us about 5 years to look at
all of the sky visible from Palomar Observatory.

Happily for us (and our families) much of the work is done by computers.
The telescope is robotically controlled and sends its data to Pasadena
every morning where it is searched by a bank of 10 computers at Caltech.
Each morning the computers find approximately 100 potentially-moving
objects that a human has to look at. The vast majority are some flaw in
the camera and are not real solar system objects, but, occasionally, as
seen above, a real object makes its presence known.

Because the new planet is so far away it is moving slower than most of
the objects that we find. It is movng so slowly, in fact, that our
computers didn't notice it the first time around! We began a special
reanalysis a year later to specifically look for very distant objects.
This reanalysis found the new planet on January 8th 2005, almost 1 1/2
years after the initial data were obtained.

What is the real name going to be?

When a new object is discoverd the International Astronomical Union
(IAU) gives it a temporary designation based on the date it was first
seen. Thus 2003 UB313 can be decoded to tell you that the data from
which the object was discovered was obtained in the second half of
October 2003. Next, the discoverers proposed a permanent name. We have
proposed such a name and are eagerly awaiting the ruling from the IAU,
which we hope will come soon. Until that time, however, we are obligated
to keep our name to ourselves. But we are free to discuss naming
philosophies!

As with everything else in this universe, there are official rules
whereby a new body in the solar system must be named. Interestingly, it
is not entirely clear which rules this new planet falls under, since no
one expected to find new planets so no rules actually exist. If the
object falls under the rules for other Kuiper belt objects, however, it
must be named after some figure in a creation mythology. We have decided
to attempt to follow that ruling scheme.

All of the other planets are named for Greek or Roman gods, so an
obvious suggestion is to attempt to find such a name for the new planet.
Unfortunately, most of the names of Greek or Roman god names
(particularly those associated with creation, which tend to be the major
gods) were used back when the first asteroids were being discovered. If
a name is already taken by an asteroid the IAU would not allow that name
to be used again. One such particularly apt name would have been
Persephone <http://www.pantheon.org/articles/p/persephone.html>. In
Greek mythology Persephone is the (forcibly abducted) wife of Hades
(Roman Pluto) who spends six months each year underground. The mourning
of her mother Demeter causes the dead of winter. The new planet is on an
orbit that could be described in similar terms; half of the time in the
vicinity of Pluto and half of the time much further away. Sadly, the
name Persephone was used in 1895 as a name for the 399th known asteroid.
The same story can be told for almost any other Greek or Roman god of
any consequence. One exception to this depletion is the Roman god Vulcan
(Greek Haphaestus), the god of fire. Astronomers have long reserved that
term, however, for a once hypothetical (now know to be nonexistent)
planet interior to Mercury (god of fire, near the sun, good name). We
would not want to use such a name to describe such a cold body!

Luckily, the world is full of mythological and spiritual traditions. In
the past we have named Kuiper belt objects after native American, Inuit,
and [minor] Roman gods. Our new proposed name expands to different
traditions, still. We hope it is accepted by the IAU and hope afterwards
that it is embraced by all.


Is this object really a planet? Is Pluto a planet? What makes a planet?

Even after all of these years of debate on the subject of whether or not
Pluto should be considered a planet, astronomers appear no closer to
agreement. (I wrote extensively about this at the time of the discovery
of Sedna. My thoughts have evolved since then, so it might be amusing to
see what I said 1 1/2 years ago. I have
been heavily influenced by writing a scientific review article this
summer on the topic of "What is a planet?" with my colleague Gibor Basri
at U.C. Berkeley who I thank for his insights) The main stumbling block
is that, scientifically, it is quite clear that Pluto should certainly
not be put in the same category as the other planets. Some astronomers
have rather desperately attempted to concoct solutions which keep Pluto
a planet, but none of these are at all satisfactory, as they also
require calling dozens of other objects planets.

Culturally, however, the idea that Pluto is a planet is enshrined in a
million different ways, from placemats of the solar system for todlers
that include the nine planets, to official NASA web sites, to mnenomics
that all school children learn to keep the nine planets straight, to
U.S. postage stamps. Our culture has fully embraced the idea that Pluto
is a planet and scientists have for the most part not yet realized that
the term planet no longer belongs to them, but, quite clearly, it
doesn't. It is understandably hard for scientists to let go of a word
that they think they use scientifically (and even in job titles like
mine: "Professor of Planetary Astronomy"!) but they need to.

>From now on, everyone should ignore the distracting debates of the
scientists, and planets in our solar system should be defined not by
some attempt at forcing a scientific definition on a
thousands-of-years-old cultural term, but by simply embracing culture.
Pluto is a planet because culture says it is.

We are then left with two cultural choices. (1) Draw the line at Pluto
and say there are no more planets; or (2) Draw the line at Pluto and say
only things bigger are planets. Both would be culturally acceptable, but
to me only the second makes sense. In addition, the second continues to
allow the possibility that exploration will find more and more planets,
which is a much more exciting prospect than that suggested by the first
possibility.

Thus, we declare that the new object, with a size larger than Pluto is
indeed a planet. A cultural planet, a historical planet. I will not
argue that it is a scientific planet, because there is no good
scientific definition which fits our solar system and our culture, and I
have decided to let culture win this one. We scientists can continue our
debates, but I hope we are generally ignored.

What else is out there?

The last week of July 2005 was an exciting one for the outer solar
system. In the course of two days the existence of three new objects was
annouced, and each object was brighter than all of the previously known
objects in the Kuiper belt (with the exception of Pluto). The first
object, 2003 EL61 <../2003EL61>, was announced by a team from Spain. The
second two, this planet and another new object named 2005 FY9, were
announced from our survey the next day. With so many bright objects
coming out at once it is hard to keep them all straight. Here is the
quick summary:

object 2003 UB313 2003 EL61 2005 FY9
discoverers Brown, Trujillo, Rabinowitz Ortiz et al. Brown, Trujillo, Rabinowitz
size bigger than Pluto! ~1/2 Pluto ~1/2 Pluto
brightness 4th brightest (KBO) 3rd brightest KBO 2nd brightest KBO

(note that though we consider Pluto and 2003 UB313 planets, they are
also clearly members of the Kuiper belt, with Pluto the brightest member)

current distance 97 AU 52 AU 52 AU
orbital period 560 years 285 years 307 years
closest approach
to sun 38 AU 35 AU 39 AU
furthest from sun 97 AU 52 AU 52 AU
tilt of orbit
compared to planets 44 degrees 28 degrees 29 degrees
satellite? unknown yes! no
surface composition Pluto-like water ice Pluto-like
when visible late summer, later winter, spring, early summer
                  fall, early winter
Received on Sun 31 Jul 2005 10:16:43 PM PDT